1. Field of the Invention
This invention relates to the cooling down of a superconducting magnet surrounded by a fluid at low pressure by a refrigerator circulating a fluid at a higher pressure.
2. Background Information
Superconducting magnets that are used in MRI cryostats and which operate in a bath of liquid helium are cooled down and tested in the factory before being shipped. It has been standard practice to leave enough helium in the magnet so that it stays cold during the three to five weeks it typically takes to get from the factory to the site where it will be used. Much of the helium boils off during transit and has to be replaced at the final site. The growing shortage of helium is motivating manufacturers to use different strategies to conserve helium. One strategy is to recover the helium from the magnet after it is tested and allow the magnet to warm up and be shipped warm. On site the magnet is then cooled down by a refrigerator that consists of a compressor external to a refrigerator cryostat in which helium is cooled then circulated through the magnet cryostat through vacuum jacketed transfer lines. A refrigerator that operates on the Brayton cycle has been developed to cool the magnet on site. It consists of a compressor that supplies gas at a discharge pressure of about 2 MPa to a counterflow heat exchanger, from which gas is admitted to an expansion space through an inlet valve, expands the gas adiabatically to about 0.8 MPa, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through vacuum jacketed transfer lines to the magnet cryostat, then returns the gas through the counterflow heat exchanger to the compressor. An MRI cryostat that can be cooled by helium at pressures as high as 1 MPa has been developed recently. Most of the MRI magnets built to date though have been designed to operate with the helium at atmospheric pressure. 0.1 MPa, and to withstand a maximum pressure of about 0.2 MPa. The object of this invention is to provide a means to cool down a magnet that can only tolerate a pressure of less than 0.2 MPa with the output from a Brayton cycle refrigerator at about 0.8 MPa.
Patent Application Publication US 2011/0219810 filed Mar. 3, 2011 by R. C. Longsworth describes a reciprocating expansion engine operating on a Brayton cycle in which the piston has a drive stem at the warm end that is driven by a mechanical drive, or gas pressure that alternates between high and low pressures, and the pressure at the warm end of the piston in the area around the drive stem is essentially the same as the pressure at the cold end of the piston while the piston is moving. Patent Application Publication US 2012/0085121 filed Oct. 4, 2011 by R. C. Longsworth describes the control of a reciprocating expansion engine operating on a Brayton cycle, as described in the previous application, that enables it to minimize the time to cool an MRI magnet to cryogenic temperatures. Patent Application Publication US 2012/0285181 filed May 12, 2011 by S. Dunn et al describes means of controlling gas flow to the warm end of the Brayton cycle engine described in the 2011/0219810 application. These engines run on a Brayton cycle that is reasonably efficient operating at a high pressure of 2 MPa and a low pressure of 0.8 MPa but would not be very efficient if the low pressure were 0.1 MPa. The best way to use this type of engine is to have helium circulate in a second cooling circuit at about 0.1 MPa that transfers heat from the magnet to a heat exchanger cooled by the Brayton cycle refrigerator.
One approach to cooling down an MRI magnet using helium at about 0.1 MPa is described in U.S. Pat. No. 6,923,009 by Kudaravalli. This system comprises a circulator at room temperature, a counter-flow heat exchanger that precools the supply gas with the return gas, a heat exchanger cooled by liquid nitrogen, and lines that enable the cold gas to flow through the magnet. U.S. Pat. No. 6,347,522 by J. F. Maguire et al describes a system for cooling a remote thermal load comprising one or more refrigerators cooling one or more cold heat exchangers, a secondary circuit of helium at about 0.1 MPa that is cooled in the cold heat exchangers, a circulator in the secondary circuit which is in the refrigerator cryostat, and lines that enable the cold gas to flow through the remote thermal load, e.g. a magnet. U.S. Pat. No. 6,625,992 by J. F. Maguire et al is a continuation of the previous patent that removes the restriction that the circulator be in the refrigerator cryostat. Patent Application Publication US 2007/0214821 filed Mar. 16, 2007 by E. Astra describes an MRI magnet that has a refrigerator mounted in the neck tube of an MRI cryostat such that the cold end is in contact with helium gas that cools the magnet, cooled helium being circulated by natural convection or one of several types of fans. U.S. Pat. No. 5,461,873 by R. C. Longsworth describes a refrigerator that is mounted in the neck tube at the top of an MRI cryostat that has tubes arranged such that the magnet is cooled by natural convection. U.S. Pat. No. 4,484,458 by R. C. Longsworth describes a refrigerator that is mounted in the neck tube at the top of an MRI cryostat and has a finned-tube heat exchanger at the cold end which circulates helium by condensing it and having it drip down.
Cooling systems that use gaseous helium at near atmospheric pressure to cool superconducting MRI magnets or other objects can provide temperatures of 4.3 K or higher. Most MRI magnets today are kept cold, after they are cooled down, by a refrigerator in the neck tube, or parallel to the neck tube, that provides cooling of about 40 W at about 40 K plus about 1 W at 4.2 K. In contrast, the refrigerator that has been designed to cool down an MRI magnet using a Brayton cycle engine as described in Patent Application Publication US 2012/0285181 produces over 1,500 W of refrigeration at 250 K and over 500 W at 100 K. This refrigerator is too large to fit in the neck tube of an MRI cryostat however instead of delivering cold helium at 0.8 MPa to the magnet the cold helium can be circulated through a heat exchanger in the neck tube which can then be used to cool the helium in the magnet at about 0.1 MPa. The heat exchanger is inserted in the neck tube before cool down and removed after the MRI magnet has been cooled to about 50 K. If the neck tube is too small to contain the heat exchanger then it can be housed in a separate heat exchanger cryostat that is removeably inserted in the smaller neck tube.